Fuel injection device for an internal combustion engine

ABSTRACT

A fuel injection system for an internal combustion engine and having a high-pressure fuel pump and a fuel injection valve for each engine cylinder. A first electrically actuated control valve controls a connection of the pump working chamber to a low-pressure region and a second electrically actuated control valve controls a connection of a control pressure chamber of the fuel injection valve to a relief region. A pressure reservoir is filled by the high-pressure fuel pump and from which fuel can be withdrawn in order to execute a fuel injection with the fuel injection valve independent of the delivery from the high-pressure fuel pump. The connection between the fuel pump and the pressure reservoir and injection valve contains a coupling device having a piston acted on at one end by the pressure prevailing in the pressure reservoir acted on at the other end by the pressure prevailing in the connection. The piston executes a delivery stroke oriented toward the pressure chamber in order to execute a fuel injection, and that the coupling device contains a bypass connection via which the connection communicates with the pressure reservoir.

PRIOR ART

The invention is based on a fuel injection system for an internalcombustion engine according to the preamble to claim 1.

A fuel injection system of this kind is known from DE 101 32 732 A. Thisfuel injection system has a respective high-pressure fuel pump and afuel injection valve connected to it for each cylinder of the internalcombustion engine. The high-pressure fuel pump has a pump piston that isdriven into a stroke motion and delimits a pump working chamber. Thepump working chamber can be connected to a low-pressure region via aconnection controlled by a first electrically actuated control valve.The fuel injection valve has an injection valve element that controls atleast one injection opening and that is acted on in an opening directionby the pressure prevailing in a pressure chamber connected to the pumpworking chamber. An electrical control element controls an opening andclosing motion of the injection valve element. The fuel injection valvehere has a control pressure chamber that can be connected to the pumpworking chamber and can also be connected to a relief region via aconnection controlled by the control element, which is embodied as asecond electrically actuated control valve. A pressure reservoir is alsoprovided into which fuel is delivered by the high-pressure fuel pump andwhich is connected to the pressure chamber of the fuel injection valve.Fuel can be drawn from the pressure reservoir for an injection,independent of the delivery by the high-pressure fuel pump. Inparticular, this permits a secondary injection of high-pressure fuel,which can occur at a time when the high-pressure fuel pump has alreadystopped delivering fuel. A secondary injection of this kind isadvantageous for reducing emissions of the engine, especiallyparticulate emissions. The connection of the pump working chamber andpressure chamber to the pressure reservoir contains a throttlerestriction and, parallel to this, a check valve that opens toward thepressure chamber. A filling of the pressure reservoir with fuel occursonly via the throttle restriction, which must be large enough to permita sufficient filling of the pressure reservoir even when the pressuregenerated by the high-pressure fuel pump is not very high and when thefuel injection quantity is low. In addition, after the termination offuel injection, a high pressure must be maintained in the pressurechamber in order to be able to deliver a large fuel quantity into thepressure reservoir, which requires a large amount of driving work fromthe high-pressure fuel pump, thus resulting in a poor efficiency of thefuel injection system. Because of the significant pressure differencesbetween the pressure reservoir on the one hand and the pump workingchamber and pressure-relieved pressure chamber on the other, anexpensively designed check valve is required in order to assure areliable seal between them.

ADVANTAGES OF THE INVENTION

The fuel injection system according to the invention, with the featuresaccording to claim 1, has the advantage over the prior art that thecoupling device with the piston permits a simply designed connection ofthe pressure reservoir to the pressure chamber and pump working chamberand does not require a sealing seat. For a fuel injection independent ofthe delivery by the high-pressure fuel pump, the piston executes adelivery stroke oriented toward the pressure chamber.

Advantageous embodiments and modifications of the fuel injection systemaccording to the invention are disclosed in the dependent claims. Theembodiment according to claim 2 or 3 provides a simple bypassconnection. In the embodiment according to claim 4, the bypassconnection can be embodied with a small flow cross section since thepressure reservoir is also filled by means of the stroke of the pistonof the coupling device. The modification according to claim 6 assuresthat the piston assumes a definite starting position from which thepiston executes a stroke for fuel delivery into the pressure reservoiror a delivery stroke toward the pressure chamber. The embodimentaccording to claim 7 likewise assures that the piston assumes a definitestarting position from which the piston executes a delivery stroketoward the pressure chamber; the pressure reservoir is filled only viathe bypass connection.

DRAWINGS

Several exemplary embodiments of the invention are depicted in thedrawings and will be explained in detail in the description thatfollows.

FIG. 1 is a schematic depiction of a fuel injection system for aninternal combustion engine according to a first exemplary embodiment,

FIG. 2 shows a detail of the fuel injection system according to a secondexemplary embodiment, and

FIG. 3 shows a detail of the fuel injection system according to a thirdexemplary embodiment.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIGS. 1 to 3 show a fuel injection system for an internal combustionengine of a motor vehicle. The fuel injection system has a respectivehigh-pressure fuel pump 10 and a fuel injection valve 12 connected to itfor each cylinder of the engine. The high-pressure fuel pump 10 and thefuel injection valve 12 can be combined into a single component, thuscomprising a so-called unit injector. Alternatively, the high-pressurefuel pump 10 and the fuel injection valve 12 can also be disposedseparate from each other and connected via a line, thus comprising aso-called unit pump.

The high-pressure fuel pump 10 has a pump piston 18 that is guided in asealed fashion in a cylinder bore 16 of a pump body 14 and is driveninto a stroke motion by a cam 20 of a camshaft of the engine, counter tothe force of a return spring 19. In the cylinder bore 16, the pumppiston 18 delimits a pump working chamber 22, in which the pump piston18 compresses fuel at high pressure. The pump working chamber 22 issupplied with fuel from a fuel tank 24 via a connection 21, for exampleby means of a fuel supply pump 25. The connection 21 of the pump workingchamber 22 to the fuel supply pump 25 contains a first electricallyactuated control valve 60. The control valve 60 is embodied as a 2/2-wayvalve and is triggered by an electronic control unit 62. The controlvalve 60 has an actuator 61 that can be an electromagnet or apiezoelectric actuator.

The fuel injection valve 12 has a valve body 26 that can be comprised ofa number of parts and contains a piston-shaped injection valve element28 that is guided so that it can slide longitudinally in a bore 30. Inits end region oriented toward the combustion chamber of the enginecylinder, the valve body 26 has at least one, preferably severalinjection openings 32. In its end region oriented toward the combustionchamber, the injection valve element 28 has a for example approximatelyconical sealing surface 34 that cooperates with a valve seat 36 embodiedin the valve body 26; the injection openings 32 branch off either fromthis valve seat or from downstream of it. In the valve body 26, betweenthe injection valve element 28 and the bore 30 toward the valve seat 36,there is an annular space 38 that transitions via a radial expansion ofthe bore 30 into a pressure chamber 40 that encompasses the injectionvalve element 28. The injection valve element 28 has a pressure shoulder42 in the region of the pressure chamber 40. The end of the injectionvalve element 28 oriented away from the combustion chamber is engaged bya prestressed closing spring 44, which presses the injection valveelement 28 toward the valve seat 36. The closing spring 44 is containedin a spring chamber 46 of the valve body 26, which adjoins the bore 30.The spring chamber 46 is connected to a relief region that can, forexample, be a return to the fuel tank 24. At its end oriented away fromthe bore 30, the spring chamber 46 can be adjoined in the valve body 26by another bore 48 in which a control piston 50 that is connected to theinjection valve element 28 is guided in a sealed fashion. With its endsurface oriented away from the spring chamber 46, the control piston 50delimits a control pressure chamber 52 in the valve body 26.

A connection 13 leading from the pump working chamber 22 feeds into thepressure chamber 40 in the valve body 26. The connection 13 that leadsbetween the pump working chamber 22 and the first control valve 60contains a check valve 53 that opens toward the pressure chamber 40. Thecheck valve 53 permits a fuel delivery from the high-pressure fuel pump10 into the pressure chamber 40, but prevents a return flow of fuel fromthe pressure chamber 40 into the pump working chamber 22 or to the fuelsupply pump 25 when the first control valve 60 is open. Upstream of thepressure chamber 40, a connection 54 that contains a throttlerestriction 55 leads from the connection 13 into the control pressurechamber 52. In addition, a connection 57 that contains a throttlerestriction 58 leads from the control pressure chamber 52 to a reliefregion, for example a return to the fuel tank 24. The connection 57contains a second electrically actuated control valve 64, which isembodied as a 2/2-way valve and is controlled by the control unit 62.The second control valve 64 has an actuator 65, which can be anelectromagnet or a piezoelectric actuator. The pressure prevailing inthe control pressure chamber 52 acts on the injection valve element 28in the closing direction in addition to the closing spring 44. Thesecond control valve 64 controls the pressure prevailing in the controlpressure chamber 52 by opening or closing the connection 57 to therelief region. The second control valve 64 thus constitutes anelectrical control element that controls the opening and closing motionof the injection valve element 28. When the second control valve 64 isclosed, the injection valve element 28 remains in its closed position oris moved into its closed position due to the high pressure in thecontrol pressure chamber 52. When the second control valve 64 is open,the injection valve element 28 can move into its open position as aresult of the low pressure in the control pressure chamber 52 if thereis a high enough pressure in the pressure chamber 40. In lieu ofcontrolling the opening and closing motion of the injection valveelement 28 by means of the second control valve 64, this function canalso be performed, for example, by a piezoelectric actuator, whichdirectly or indirectly acts on the injection valve element 28 in itsclosing direction. The control pressure chamber 52 and the secondcontrol valve 64 can then be eliminated.

Downstream of the check valve 53, a connection 66 to a pressurereservoir 68 branches off from the connection 13 between the pumpworking chamber 22 and the pressure chamber 40 and control pressurechamber 52. The connection 66 contains a coupling device 70, which FIG.1 depicts in accordance with a first exemplary embodiment. The couplingdevice 70 has a piston 74 that is guided so that it can slide in acylinder bore 72. The coupling device 70 has a bypass connection betweenthe two end surfaces of the piston 74, which can be embodied, forexample, as a conduit 76 extending through the piston 74. The conduit 76contains a throttle restriction 77. The bypass connection canalternatively also be embodied, as in a second exemplary embodimentdescribed below, in the form of an annular gap 176 with a small crosssection that extends between the cylinder bore 172 and the outercircumference of the piston 174. On its end surface oriented toward thepressure reservoir 68, the piston 74 is acted on by the pressureprevailing in the pressure reservoir 68 and on its end surface orientedaway from the pressure reservoir 68, is acted on by the pressureprevailing in the connection 13. The piston 74 can be moved back andforth in the cylinder bore 72 between an end position oriented towardthe pressure reservoir 68 and an end position oriented toward theconnection 13, i.e. away from the pressure reservoir 68. Preferably acommon pressure reservoir 68 is provided for all of the cylinders of theinternal combustion engine. The pressure reservoir 68 can be embodied asa separate component, for example in a tubular or spherical form.Alternatively, the pressure reservoir can also be constituted by aninternal volume of the fuel injection system or by the volume in theconnecting lines of the fuel injection system. The pressure reservoir 68can be provided with a pressure relief device 69, which limits thepressure prevailing in the pressure reservoir 68 to a predeterminedvalue. The pressure relief device 69 can be embodied as a pressurerelief valve that limits the pressure in the pressure reservoir 68 to aconstant value. Alternatively, the pressure relief device 69 can also beembodied as a control valve that can limit the pressure prevailing inthe pressure reservoir 68 in a variable fashion, for example as afunction of operating parameters of the engine, and can be triggered bythe control unit 62.

The function of the fuel injection system will be explained below.During an intake stroke of the pump piston 18, the first control valve60 is opened so that the fuel supply pump 25 delivers fuel from the fueltank 24 into the pump working chamber 22 via the connection 21. Thecheck valve 53 is closed in this instance since the pressure generatedby the fuel supply pump 25 is less than the pressure prevailing in thepressure chamber 40, the control pressure chamber 52, and the connection13 downstream of the check valve 53. During the delivery stroke of thepump piston 18, the first control valve 60 is closed so that highpressure builds up in the pump working chamber 22. If the pressure inthe pump working chamber 22 is greater than the pressure prevailing inthe pressure chamber 40 and the control pressure chamber 52, then thecheck valve 53 opens and fuel travels to the fuel injection valve 12. Ifthe second control valve is closed 64, then at least approximately thesame pressure prevails in the control pressure chamber 52 as in thepressure chamber 40 and the injection valve element 28 is kept in itsclosed position, in which it rests with its sealing surface 34 againstthe valve seat 36 and closes the at least one injection opening 32 sothat no fuel injection can occur. At a time determined by the controlunit 62 as a function of operating parameters of the engine, the controlunit 62 opens the second control valve 64 so that the control pressurechamber 52 is connected to the relief region and the pressure in thecontrol pressure chamber 52 decreases. The relief of the controlpressure chamber 52 reduces the force acting on the injection valveelement 28 in the closing direction so that the pressure prevailing inthe pressure chamber 40 moves this injection valve element 28 in theopening direction 29, thus opening the at least one injection opening 32through which fuel is injected. It is possible that at first, only asmall quantity of fuel is injected in a preinjection; then the controlunit 62 closes the second control valve 64 again for a short time sothat the pressure increases in the control pressure chamber 52 and theinjection valve element 28 is moved into its closed position. It is alsopossible for there to be a number of preinjections in sequence.

For a main injection of a large quantity of fuel, the control unit 62opens the second control valve 64 again for a time that corresponds tothe fuel quantity to be injected. In order to terminate the maininjection, the control unit 62 closes the second control valve 64 andopens the first control valve 60. This relieves the pump working chamber22 through the opened connection 21 to the fuel supply pump 25 so thatthe high-pressure fuel pump 10 does not deliver any more fuel. Thepressure drop in the pump working chamber 22 causes the check valve 53in the connection 13 to close. The control unit 62 closes the secondcontrol valve 64.

When the high-pressure fuel pump 10 delivers fuel to the pressurechamber 40 via the connection 13, it also delivers fuel via theconnection 66 to the coupling device 70 and into the pressure reservoir68. At the beginning of the fuel delivery by the high-pressure fuel pump10, the piston 74 of the coupling device 70 is disposed in its endposition oriented away from the pressure reservoir 68, in which thepiston 74 is depicted with dashed lines in FIG. 1. During fuel deliveryby the high-pressure fuel pump 10, the piston 74 is slid into its endposition oriented toward the pressure reservoir 68, in which the pistonis depicted with solid lines in FIG. 1, and thus executes a deliverystroke in that the fuel displaced from the cylinder bore 72 by thepiston 74 is fed into the pressure reservoir 68. In addition, fuel isalso delivered into the pressure reservoir 68 via the conduit 76 in thepiston 74, the flow through the conduit 76 being limited by the throttlerestriction 77. After the end of the main injection, an elevatedpressure is maintained in the pressure chamber 40, the control pressurechamber 52, and the connection 13 downstream of the check valve 53,which likewise results in a filling of the pressure reservoir 68 via theconduit 76 in the piston 74.

To execute one or more secondary injections, the control unit 62 opensthe second control valve 64 so as to relieve the pressure in the controlpressure chamber 52. Fuel then flows out of the pressure reservoir 68 atthe pressure prevailing in the pressure reservoir 68 and into thepressure chamber 40, permitting the injection valve element 28 to openand thus permitting an injection of fuel. In addition, the piston 74 ofthe coupling device 70 also executes a delivery stroke oriented awayfrom the pressure reservoir 68 and displaces fuel from the cylinder bore72 into the pressure chamber 40. The pressure reservoir 68 and thecoupling device 70 thus permit a fuel injection, in particular asecondary injection, independent of the fuel delivery by thehigh-pressure fuel pump 10. A secondary injection is advantageous inorder to reduce emissions, especially particulate emissions, of theengine and permits a regeneration of exhaust treatment devices such asparticulate filters or catalytic converters. An injection cycle includesat least one preinjection, a main injection, and at least one secondaryinjection.

At the beginning of the next injection cycle, the piston 74 of thecoupling device 70 is then disposed, as explained above, in a positionoriented away from the pressure reservoir 68 and during fuel delivery bythe high-pressure fuel pump 10, moves into its end position orientedtoward the pressure reservoir 68.

FIG. 2 shows a detail of the fuel injection system according to a secondexemplary embodiment, in which the basic design is the same as in thefirst exemplary embodiment and only the coupling device 170 has beenmodified. The coupling device 170 has the cylinder bore 172 in which thepiston 174 is guided in a sliding fashion. The bypass connection isconstituted by a small diameter annular gap 176 between the cylinderbore 172 and the outer circumference of the piston 174, which annulargap also constitutes a throttle restriction. In the second exemplaryembodiment, however, the bypass connection can also be embodied the sameas in the first exemplary embodiment, in the form of a conduit thatcontains a throttle restriction and extends through the piston 174.Spring elements 178 and 180 that are embodied as helical compressionsprings engage the piston 174 at both ends. The spring 178 that engagesthe end surface of the piston 174 oriented toward the pressure reservoir68 acts on the piston 174 in the direction oriented away from thepressure reservoir 68 and the spring 180 that engages the end surface ofthe piston 174 oriented away from the pressure reservoir 68 acts on thepiston 174 in the direction oriented toward the pressure reservoir 68.Between two successive injection cycles, the two springs 178, 180 holdthe piston 174 in a middle position depicted with solid lines in FIG. 2,between its two end positions. During fuel delivery into the pressurereservoir 68 as part of an injection cycle, the piston 174 is slid fromits middle position into its end position oriented toward the pressurereservoir 68. The piston 174 remains in this end position until awithdrawal of fuel from the pressure reservoir 68 produces a secondaryfuel injection in which the piston 174 is slid past its middle positioninto its end position oriented away from the pressure reservoir 68.After the end of the secondary injection and therefore after aninjection cycle, the springs 178, 180 move the piston 174 back into itsmiddle position. At the beginning of fuel delivery by the high-pressurefuel pump 10 during the next injection cycle, the piston 174 istherefore always disposed in its definite middle position, which is itsstarting position. The remaining functions of the fuel injection systemaccording to the second exemplary embodiment are the same as in thefirst exemplary embodiment.

FIG. 3 shows the fuel injection system according to a third exemplaryembodiment in which once again, only the coupling device 270 has beenmodified in relation to the first exemplary embodiment. The couplingdevice 270 has the cylinder bore 272 in which the piston 274 is guidedin a sliding fashion. The piston 274 contains the bypass conduit 276with the throttle restriction 277. Alternatively, the bypass conduit canalso be embodied as in the second exemplary embodiment, in the form ofan annular gap between the piston 274 and the cylinder bore 272. Aspring element 280 in the form of a helical compression spring engagesthe end surface of the piston 270 oriented away from the pressurereservoir 68 and acts on the piston 274 in the direction of its endposition oriented toward the pressure reservoir 68. Between twosuccessive injection cycles, the spring 280 holds the piston 274 in itsend position oriented toward the pressure reservoir 68, which positionis depicted with solid lines in FIG. 3. During an injection cycle, fuelis delivered into the pressure reservoir 68 only via the conduit 276;the throttle restriction 277 must be large enough to permit a sufficientfilling of the pressure reservoir 68. The piston 274 remains in this endposition until a withdrawal of fuel from the pressure reservoir 68results in a secondary injection of fuel during which the piston 274 isslid into its end position oriented away from the pressure reservoir 68.After the end of the secondary injection and therefore after aninjection cycle, the spring 280 moves the piston 274 back into its endposition oriented toward the pressure reservoir 68. At the beginning offuel delivery by the high-pressure fuel pump 10 in the next injectioncycle, the piston 274 is therefore always disposed in its definite endposition oriented toward the pressure reservoir 68, which is itsstarting position. The remaining functions of the fuel injection systemaccording to the second exemplary embodiment are the same as in thefirst exemplary embodiment.

1-8. (canceled)
 9. A fuel injection system for an internal combustionengine, comprising a respective high-pressure fuel pump (10) and a fuelinjection valve (12) connected to it for each cylinder of the internalcombustion engine, the high-pressure fuel pump (10) having a pump piston(18) driven into a stroke motion and delimiting a pump working chamber(22), which can be connected to a low-pressure region (25) via aconnection (21) that is controlled by an electrically actuated controlvalve (60), the fuel injection valve (12) having an injection valveelement (28) that controls at least one injection opening (32) and isacted on in an opening direction (29) by the pressure prevailing in apressure chamber (40), which can be connected to the pump workingchamber (22), an electrical control element (64) controlling an openingand closing motion of the injection valve element (28), a pressurereservoir (68) communicating with the pump working chamber (22) via aconnection (66) through which fuel is delivered into the pressurereservoir (68) during the delivery stroke of the pump piston (18) andalso communicating with the pressure chamber (40) of the fuel injectionvalve (12) via the connection (66) through which the pressure chamber(40) can be supplied with fuel from the pressure reservoir (68) for afuel injection via the fuel injection valve (12) independent of thedelivery stroke of the pump piston (18), the connection (66) of thepressure reservoir (68) to the pump working chamber (22) and pressurechamber (40) containing a coupling device (70; 170; 270), which containsa sliding piston (74; 174; 274) that is acted on at one end by thepressure prevailing in the pressure reservoir (68) and is acted on atthe other end by the pressure prevailing in the connection (66), thepiston (74; 174; 274) executing a delivery stroke oriented toward thepressure chamber (40) in order to execute a fuel injection, and thecoupling device (70; 170; 270) containing a bypass connection (76, 77;176; 276, 277) via which the connection (66) communicates with thepressure reservoir (68).
 10. The fuel injection system according toclaim 9, wherein the bypass connection comprises a conduit (76; 176;276) that extends through the piston (74; 174; 274) and contains athrottle restriction (77; 177; 277).
 11. The fuel injection systemaccording to claim 9, wherein the bypass connection comprises of aconduit (176) that is formed between the outer circumference of thepiston (174) and a cylinder bore (172) in which the cylinder (174) isguided.
 12. The fuel injection system according to claim 9, wherein thepiston (74; 174) executes a stroke oriented toward the pressurereservoir (68) in order to fill the pressure reservoir (68).
 13. Thefuel injection system according to claim 10, wherein the piston (74;174) executes a stroke oriented toward the pressure reservoir (68) inorder to fill the pressure reservoir (68).
 14. The fuel injection systemaccording to claim 11, wherein the piston (74; 174) executes a strokeoriented toward the pressure reservoir (68) in order to fill thepressure reservoir (68).
 15. The fuel injection system according toclaim 12, wherein the piston (74; 174; 274) can be moved between adefinite end position oriented toward the pressure reservoir (68) and adefinite end position oriented toward the connection (66).
 16. The fuelinjection system according to claim 13, wherein the piston (74; 174;274) can be moved between a definite end position oriented toward thepressure reservoir (68) and a definite end position oriented toward theconnection (66).
 17. The fuel injection system according to claim 14,wherein the piston (74; 174; 274) can be moved between a definite endposition oriented toward the pressure reservoir (68) and a definite endposition oriented toward the connection (66).
 18. The fuel injectionsystem according to claim 12, further comprising at least one springelement (178, 180; 280) acting on the piston (174; 274) in the directionof at least one end position.
 19. The fuel injection system according toclaim 13, further comprising at least one spring element (178, 180; 280)acting on the piston (174; 274) in the direction of at least one endposition.
 20. The fuel injection system according to claim 14, furthercomprising at least one spring element (178, 180; 280) acting on thepiston (174; 274) in the direction of at least one end position.
 21. Thefuel injection system according to claim 12, further comprising twospring elements (178, 180) one acting on the piston (174) in thedirection toward opposite end positions, and wherein between twosuccessive injection cycles, the spring elements (178, 20) hold thepiston (174) in a definite intermediate position between the two endpositions.
 22. The fuel injection system according to claim 13, furthercomprising two spring elements (178, 180) one acting on the piston (174)in the direction toward opposite end positions, and wherein between twosuccessive injection cycles, the spring elements (178, 20) hold thepiston (174) in a definite intermediate position between the two endpositions.
 23. The fuel injection system according to claim 14, furthercomprising two spring elements (178, 180) one acting on the piston (174)in the direction toward opposite end positions, and wherein between twosuccessive injection cycles, the spring elements (178, 20) hold thepiston (174) in a definite intermediate position between the two endpositions.
 24. The fuel injection system according to claim 12, furthercomprising a spring element (280) holding the piston (274) in its endposition oriented toward the pressure reservoir (68) between twosuccessive injection cycles.
 25. The fuel injection system according toclaim 13, further comprising a spring element (280) holding the piston(274) in its end position oriented toward the pressure reservoir (68)between two successive injection cycles.
 26. The fuel injection systemaccording to claim 14, further comprising a spring element (280) holdingthe piston (274) in its end position oriented toward the pressurereservoir (68) between two successive injection cycles.
 27. The fuelinjection system according to claim 9, further comprising a pressurerelief device (69) which limits the pressure in the pressure reservoir(68) to a predetermined value.